Field of the Invention
The present disclosure relates to an image forming apparatus for performing image forming processing by electrophotography, such as a copying machine, a printer, a recording image display device, a facsimile and the like.
Description of the Related Art
The electrophotographic system image forming apparatus forms color images by forming electrostatic images by irradiating laser light to each of the photoreceptors provided for every color; adhering the toner of the corresponding color to each electrostatic image to form the toner images of each color, and sequentially overlapping and transferring the toner images of each color onto the transfer belt. The laser light is reflected by a rotary reflecting mirror such as a polygon mirror and scans the photoreceptor by the rotation of the rotary reflecting mirror. Due to the changes in an ambient environment including temperature or humidity, printing characteristics such as color tones or gradation changes, which cause position registration between the images of each color. Thus, the image forming apparatus forms a test image on the transfer belt, which is the image used to measure density registration or position registration of the image. By detecting the test image formed by a sensor, the image forming apparatus obtains density information representing the density of the image or position registration information representing the position registration amount. The image forming apparatus adjusts the image by correcting the density of the image based on the density information obtained and the position registration of the images of each color (hereinafter referred to as “color registration correction”) based on the position registration information obtained. In US2007/0025779(A1), such image adjustment is performed for every time the image formation is performed to a predetermined number of sheets.
The image forming apparatus is designed so as not to convey sheets in a skewed manner, however, it is difficult to completely suppress skewed feeding of the sheet. As a result, the sheet on which the image is formed may be conveyed in a skewed manner. When the image is transferred from the transfer belt to the skewed sheet, the image formed on the sheet is skewed. To solve this, according to the amount of the skew of the sheet, the image is inclined and transferred to the sheet. In this case, by correcting the image data representing the image, the image to be formed is inclined according to the amount of skew of the sheet.
When the skewed feeding of the sheet is corrected by inclining the image, writing start timing of the image in a sub-scanning direction is changed according to the amount of the skew. The processing to correct the image data to incline the image is also performed when correcting the color registration of the images of each color. By performing the color registration correction, the amount of the inclination of the image of each color with respect to the amount of the skew varies. Due to this, the writing start timing in the sub-scanning direction differs.
There may be a case where a timing to form a test image for detecting the color registration (color registration correction test image) does not correspond to a timing to form a test image for detecting density. Regardless of whether the sheet is skewed or not, it is necessary to detect the degree of registration between the colors in a color registration detection control. Thus, the image forming apparatus first controls phase relation of one or more rotating polygon mirrors such to be in a predetermined phase relationship. Then, the image forming apparatus forms the color registration correction test image. The predetermined phase relation is the phase relation having no relationship to data relating to the amount of skew of the sheet. Thus, time to control the phase relation of one or more rotating polygon mirror to be in a predetermined phase relation is provided. By forming the color registration correction test image with the phase relation of one or more rotating polygon mirrors in a predetermined relation and using the detection result and the data relating to the amount of skew, it becomes possible to correct the color registration by a unit of less than one pixel.
On the other hand, in case of the test image for detecting density (density correction test image), detecting the density is only required. Thus, even the image is formed at a position registrated from an ideal position by less than one pixel or a few pixels, the detection result will not change. As mentioned, the phase relation of one or more rotating polygon mirror is controlled when forming the color registration correction test image. If the phase relation of one or more rotating polygon mirror is similarly controlled when forming the density correction test image, time to start forming the next image is delayed. This is because time is taken to control the phase of one or more rotating polygon mirrors. The present disclosure provides the image forming apparatus with less standby time and enhanced productivity.